The invention relates to a method of writing information in a data storage medium comprising an optical information medium, and also a data storage medium.
DE 298 16 802 describes a data storage medium comprising an optical information medium which contains a polymer carrier in the form of a polymer film. Cited as a material for the polymer film is polymethyl methacrylate, and also a polymer film which is marketed by Beiersdorf AG under the designation xe2x80x9ccrystal-clear Tesafilmxe2x80x9d, which has biaxially oriented polypropylene. In the case of this data storage medium, the polymer film is wound spirally in a plurality of plies on to a winding core, there being an adhesion layer in each case between adjacent plies. Information can be written in the data storage medium by the polymer film being heated locally with the aid of a write beam from a data drive. This is because the energy stored in the polypropylene as a result of biaxial stretching during film production is liberated again during the local heating by the write beam (short laser pulses), the polymer film material contracting locally and therefore changing its refractive index at those locations exposed to the write beam. This leads to a local change in the reflective power (the reflectivity) at the interface of the polymer film, which can be registered with the aid of a read beam in the data drive. By means of focusing the write beam or read beam, information can be written specifically into a preselected ply of the information medium or can be read out from it. The winding core can be optically transparent and, at its center, have a recess, which is used to accommodate the writing and reading device of a data drive. In this case, the writing and reading device is moved relative to the data storage medium, while the data storage medium is stationary, so that the data storage medium need not be balanced to take account of rapid rotational motion.
In order to convert the output of the write beam effectively into heat and, in this way, to achieve a local change in the refractive index of the order of magnitude of 0.2, which is sufficient to store information, an absorber, which is contained in the adhesion layer, is used in the data storage medium previously disclosed. However, the absorber is disadvantageous, since it also attenuates the read beam. This has a disruptive effect, in particular in the case of multi-ply systems like the data storage medium previously disclosed. In addition, it is desirable for the achievable change in the refractive index to be still greater, in order to obtain a stronger signal from the read beam.
It is an object of the invention to provide improved options for a data storage medium comprising an optical information medium which has a polymer carrier.
This object is achieved by a method of writing information in a data storage medium comprising an optical information medium having the features of claim 1, and also by data storage media having the features of claims 11 and 12. Advantageous refinements of the invention emerge from the dependent claims.
The method according to the invention is used to write information in a data storage medium comprising an optical information medium which has a polymer carrier. In this case, atoms and/or molecules that change the refractive index are introduced into the polymer carrier, at individual locations associated with information units, as a function of the information to be entered.
The atoms and/or molecules that change the refractive index and are located in the area of the polymer carrier envisaged for the storage of an information unit effect a change in the refractive index. The result of this is a local change in the reflective power (the reflectivity) at the interface or at the interfaces of the polymer carrier with an adjacent medium. This can be registered with the aid of a read beam which, at the location considered, is reflected as a function of the information entered, that is to say the local content of atoms and/or molecules that change the refractive index. As a result of introducing the atoms and/or molecules that change the refractive index into the polymer carrier, the optical properties of the polymer carrier can be changed effectively and in a defined way. For example, local changes in refractive index of the order of magnitude of 0.2 and more may be achieved, which is sufficient for the data entered to be read out, for example with the aid of a read beam. A suitable polymer carrier is, for example, a polymer film.
In the polymer carrier, the information units are formed by changing the optical properties in an area having a preferred size of less than 1 xcexcm. Here, the information can be stored in a binary form, that is to say at the location of an information unit, the local reflectivity assumes only two values. In other words, if the reflectivity is above a defined threshold value, a xe2x80x9c1xe2x80x9d, for example, is stored at the considered location on the information medium and if it lies below this threshold value or below a different, lower threshold value, it is accordingly a xe2x80x9c0xe2x80x9d. However, it is also conceivable to store the information in a plurality of gray stages. This is possible if the optical properties of the polymer carrier, at the location of an information unit, can be changed in a specific way by means of defined setting of the refractive index without saturation being reached in the process.
In an advantageous refinement of the method according to the invention, the atoms and/or molecules that change the refractive index are diffused into the polymer carrier, to be specific, preferably by means of local heating. In this case, the atoms and/or molecules that change the refractive index can originate from a layer which is applied to the polymer carrier. In order to introduce the atoms and/or molecules that change the refractive index into the polymer carrier at the location of an information unit, the layer or the polymer carrier adjacent thereto is heated in the relevant area, so that the atoms and/or molecules that change the refractive index can migrate out of the layer and diffuse into the polymer carrier. Since no atoms and/or molecules that change the refractive index diffuse into the polymer carrier from adjacent areas which are not heated, in this way a two-dimensional distribution of atoms and/or molecules that change the refractive index can be arranged in the polymer carrier, said distribution corresponding to the pattern of the information to be entered.
If the rest of the layer from which the atoms and/or molecules that change the refractive index originate, is removed from the polymer carrier after the information has been entered, the signals registered by a read beam are particularly clear, since the atoms and/or molecules that change the refractive index are virtually all located in the polymer carrier. It is not therefore possible for any disruptive influence to originate from atoms and/or molecules which change the refractive index in the rest of the layer.
After the rest of the layer has been removed, however, no new or further information can be written in the data storage medium. However, it is not absolutely necessary to remove the rest of the layer from the polymer carrier after information has been entered. This is because the atoms and/or molecules that change the refractive index have, in the molecular surroundings of the polymer carrier, different optical properties than in the layer in which they are generally stored in a higher concentration and, depending on the embodiment, in a matrix. However, a part is also played by the interface between the polymer carrier and the interface of the polymer carrier opposite the layer, whose reflectivity is predominantly influenced by the atoms and/or molecules that change the refractive index and have diffused into the polymer carrier, and not by the rest of the layer located on the other side of the polymer carrier.
One other possible way of introducing the atoms and/or molecules that change the refractive index into the polymer carrier is to implant them into the polymer carrier by means of particle beams.
The information to be entered can be entered, for example, by means of a focused write beam. For example, a focused laser beam may be used as a write beam, which locally heats a layer applied to the polymer carrier and comprising atoms and/or molecules that change the refractive index, so that atoms and/or molecules that change the refractive index diffuse from there into the polymer carrier. In addition, in the case of an implantation method, a particle beam may also be used as a write beam.
One other possible way of writing the polymer carrier is to enter the information to be entered over a large area, using a mask. In this case, the polymer carrier can be placed behind a mask, which is provided with a pattern in accordance with the information to be entered. Located in front of the mask is a source, for example a thermal radiation source or a light source, whose radiation penetrates in accordance with the pattern on the mask as far as the polymer carrier or a layer comprising atoms and/or molecules that change the refractive index on the polymer carrier, or a source of particle radiation, in order to implant atoms and/or molecules that change the refractive index into the polymer carrier in accordance with the pattern on the mask.
In an advantageous refinement of the method according to the invention, the information to be entered is entered by means of irradiation with infrared light, for example by using a write beam or a mask, as already mentioned. The infrared light (thermal radiation) aimed at the area for the storage of an information unit effects local heating, which leads to diffusion of atoms and/or molecules that change the refractive index into the polymer carrier. Infrared light in the wavelength range around 1.5 xcexcm is particularly suitable, since the material of the polymer carrier (for example polypropylene, see below) specifically generally exhibits relatively high absorption there, which is caused by harmonics in the C-H stretching vibrations.
Conceivable atoms and/or molecules that change the refractive index are a large number of different atoms and/or molecules. The selection depends, for example, on the compatibility with the polymer carrier, the magnitude of the effect to be achieved, (that is to say the desired change in the refractive index), the optical properties in the spectral range of the read beam used to read the information, and so on.
It is particularly advantageous to use highly polarizable molecules as atoms and/or molecules that change the refractive index. They have a relatively high refractive index and therefore influence the optical properties of the polymer carrier relatively strongly if they are introduced there.
Particularly suitable highly polarizable molecules are halogen-containing molecules. For example, chlorine and bromine, with their high polarizability, increase the refractive index. Halogen-containing materials considered for a layer comprising molecules that change the refractive index and applied to the polymer carrier are, in particular, resins and oligomers. In the range of acrylates, in particular, there exists a large number of commercially obtainable, partially or completely halogenized monomers.
In addition, aromatic molecules may be used as highly polarizable molecules. The refractive index of hydrocarbons can be set by the aromaticity; aromatics have considerably higher refractive indices than saturated hydrocarbons. Particularly large effects may be achieved with halogen-containing aromatic molecules.
Another possibility is to use slightly polarizable molecules as atoms and/or molecules that change the refractive index, that is to say molecules which, as compared with the polymer carrier, have a low polarizability and therefore a low refractive index. This is because such molecules also exhibit a relatively intense action on the optical properties of the polymer carrier, if they are introduced there to store information. For example, the refractive index of a medium decreases if hydrogen therein is replaced by fluorine.
The method according to the invention of writing information in a data storage medium may be implemented in a particularly advantageous way in connection with two types of data storage media.
One type of data storage medium according to the invention has an optical information medium with information that has already been entered and has a polymer carrier. In this case, the polymer carrier contains atoms and/or molecules that change the refractive index at individual locations associated with information units, as a function of the entered information. The data storage medium can be written by the manufacturer in one of the ways explained above. In this case, if the atoms and/or molecules that change the refractive index were diffused into the polymer carrier from a layer applied to the polymer carrier, and this layer was subsequently removed, no new or further information can be entered into the data storage medium by the user, at least not in accordance with the method according to the invention.
The second type of data storage medium according to the invention has an optical information medium which has a polymer carrier, the polymer carrier being provided with a layer which contains atoms and/or molecules that change the refractive index. These can be diffused into the polymer carrier by local heating. A data storage medium of this type can therefore be written by the user; however, the data or part of the data can also already have been entered by the manufacturer.
The atoms and/or molecules that change the refractive index preferably comprise highly polarizable molecules, such as halogen-containing molecules or aromatic molecules but also slightly polarizable molecules, as explained above.
If the polymer carrier is provided with a layer which contains atoms and/or molecules that change the refractive index, then in a preferred refinement, an absorber is assigned to the layer and is set up to absorb a write beam, at least partly, and to locally discharge, at least partly, the heat produced in the process to the layer and/or the polymer carrier. For example, the absorber can be contained in the layer, in the polymer carrier or in an adhesion layer adjacent to the polymer carrier. It makes easier the absorption of a write beam and therefore the local heating required for the diffusion of the atoms and/or molecules that change the refractive index. Alternatively, (or else additionally), the heating can also be carried out by means of irradiation with infrared light, for example in the wavelength range around 1.5 xcexcm, as already explained. This is because a polymer carrier, for example one made of polypropylene, has a relatively high absorption, caused by harmonics of the C-H stretching vibrations. Therefore, it may be possible to dispense with an additional absorber.
The information medium preferably has a plurality of polymer carrier plies, through which information units can be read from a preselected polymer carrier ply and, if appropriate, written to a preselected polymer carrier ply. In each case an adhesion layer can be arranged between adjacent polymer carrier plies, in order to fix the polymer carrier plies to one another. Suitable adhesion means are, for example, an air-bubble free acrylate adhesive which, for example, is crosslinked chemically or by means of UV or electron radiation. If the refractive index of the adhesion layer differs only slightly from the refractive index of the polymer carrier, disruptive reflections of a read beam or write beam at an interface between a polymer carrier ply and an adjacent adhesion layer are minimized. It is particularly advantageous if the difference in the refractive indices is less than 0.005. However, an existing difference in the refractive indices can be used to format the data storage medium. It is conceivable for a layer which contains atoms and/or molecules that change the refractive index to have adhesive properties (see above), so that an additional adhesion layer can be dispensed with.
Plate material can be used as the polymer carrier. However, the polymer carrier may also have a polymer film, for example made of a biaxially oriented polypropylene (BOPP). If a polymer film is used as the polymer carrier, then in a preferred embodiment, the information medium is wound on spirally, an adhesion layer preferably being provided in each case between adjacent polymer film plies. For example, 10 to 30 polymer film plies can be wound on but also more or fewer. Given a thickness of the polymer film between 10 xcexcm and 100 xcexcm, preferably below 50 xcexcm or around 35 xcexcm, the information on different polymer film plies can be mutually separated with good resolution, with the aid, for example, of reading and writing devices known from DVD technology. An adhesion layer can, for example have a thickness in the range between 1 xcexcm and 40 xcexcm, preferably below 25 xcexcm or around 2 xcexcm.
The data storage medium comprising a spiral-wound information medium preferably has an optically transparent winding core which has a recess in its central area. In this case, it is possible to arrange a reading device and, optionally, a writing device of a drive tuned to the data storage medium in the recess in the central area of the winding core, and, in order to read or write information, to move it relative to the data storage medium, while the data storage medium is stationary. A stationary data storage medium has the advantage that it does not have to be balanced in order to permit high rotational speeds, which has a beneficial effect on the manufacturing costs.